For monitoring of long-lived fission products in water a fast monitor with Čerenkov detector has been designed. The monitor responds in less than a second to Y 91 activities of 10 -4 μc/cm 3 and can be used for water flows up to about 60 1/min. Detector construction and electronic equipment are described and the selection of photomultiplier tube is discussed. The detector efficiency is given for some important isotopes as well as the minimum activity that can be detected. The performance of the monitor in an application connected with the handling of burned out fuel elements is also reported.